KR100994090B1 - Lithium secondary battery - Google Patents

Abstract

A lithium secondary battery composed of a positive electrode, a negative electrode composed of artificial graphite or natural graphite, and a nonaqueous electrolyte having an electrolyte dissolved in a nonaqueous solvent, wherein the cyclohexylbenzene having a halogen atom bonded to a benzene ring in the nonaqueous electrolyte is 0.1 to 20% by weight. It contains. Lithium secondary batteries exhibit large capacity and excellent cycle characteristics.

Lithium secondary battery

Description

Lithium Secondary Battery {LITHIUM SECONDARY BATTERY}

The present invention relates to a lithium secondary battery having excellent battery characteristics in cycle characteristics, capacitance and storage characteristics of the battery.

In recent years, lithium secondary batteries are generally used as power sources for driving small electronic devices. A lithium secondary battery essentially contains a positive electrode, a nonaqueous electrolyte, and a negative electrode. Lithium secondary batteries using a positive electrode made of a lithium composite oxide such as LiCoO ₂ , a negative electrode made of a carbon material or a lithium metal are advantageously used. As the nonaqueous electrolyte for lithium secondary batteries, carbonates such as ethylene carbonate (EC) or propylene carbonate (PC) are advantageously used.

Nevertheless, it is desirable to provide a secondary battery that exhibits improved battery characteristics in battery cycle characteristics and capacitance.

Lithium _secondary batteries using LiCoO ₂ , LiMn ₂ O _4, or LiNiO ₂ as positive electrodes sometimes exhibit poor battery performance, as some of the non-aqueous solvent in the non-aqueous electrolyte is oxidatively decomposed during charging, resulting in the desired electrochemical decomposition. This is because it interferes with the reaction. The decomposition is thought to occur by electrochemical oxidation of the solvent at the interface between the positive electrode and the nonaqueous electrolyte.

On the other hand, a lithium secondary battery using a high crystallized carbon material such as natural graphite or artificial graphite as a negative electrode also exhibits a decrease in battery performance since the solvent in the electrolyte is partially reduced on the surface of the negative electrode during charging. . Reductive decomposition also occurs during repeated charge-discharge processes when EC (generally used as the nonaqueous solvent of the electrolyte) is used as the nonaqueous solvent.

Japanese Patent Application Laid-open No. Hei 10-74537 discloses that when a small amount of an aromatic compound such as benzene having a hydrocarbon substituent (for example, cyclohexylbenzene) is added to a nonaqueous electrolyte of a lithium secondary battery, the cycle characteristics and the electric capacity of the battery are improved.

Japanese Patent Publication No. 10-112335 discloses that the cycle characteristics of a battery are improved when a small amount of a fluorine atom-containing aromatic compound such as fluorobenzene is added to a nonaqueous electrolyte of a lithium secondary battery.

Disclosure of Invention

An object of the present invention is to provide a lithium secondary battery having improved cycle characteristics, improved capacitance, and improved storage capacity in a charged state.

The present invention is a lithium secondary battery comprising a non-aqueous electrolyte having a positive electrode, a negative electrode made of artificial graphite or natural graphite, and an electrolyte dissolved in a non-aqueous solvent, wherein the non-aqueous electrolyte has a cyclohexylbenzene having a halogen atom bonded to a benzene ring. It relates to a lithium secondary battery containing 0.1 to 20% by weight.

 As used herein, cyclohexylbenzenes having a halogen atom bonded to a benzene ring are preferably compounds having the formula (I):

[Wherein X is a halogen atom and the halogen atom is attached at an arbitrary position].

 Preference is given to 1-halogeno-4-cyclohexylbenzene.

In cyclohexylbenzene (hereinafter referred to as "cyclohexyl-halogenobenzene") having a halogen atom bonded to a benzene ring contained in a nonaqueous electrolyte in which an electrolyte is dissolved in a nonaqueous solvent, the halogen atom is a fluorine atom or a chlorine atom. desirable.

Examples of cyclohexyl-halogenobenzene include 1-fluoro-2-cyclohexylbenzene, 1-fluoro-3-cyclohexylbenzene, 1-fluoro-4-cyclohexylbenzene, 1-chloro-4-cyclo Hexylbenzene, 1-bromo-4-cyclohexylbenzene, and 1-iodo-4-cyclohexylbenzene.

If the content of cyclohexyl-halogenobenzene in the nonaqueous electrolyte is too high, battery performance may be reduced. When the content of cyclohexyl-halogenobenzene is too small, the expected improvement in battery performance cannot be obtained. Therefore, in order to sufficiently improve the cycle characteristics, the content is preferably 0.1 to 20% by weight, more preferably 0.2 to 10% by weight, and most preferably 0.5 to 5% by weight based on the weight of the nonaqueous electrolyte. .

Examples of the nonaqueous solvent used in the nonaqueous electrolyte of the present invention include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and vinylene carbonate (VC); Lactones such as γ-butyrolactone; Linear carbonates such as dimethyl carbonate (DMC), methyl ethyl carbonate (MEC) and diethyl carbonate (DEC); Ethers such as tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, and 1,2-dibutoxyethane; Nitriles such as acetonitrile and adipononitrile; Esters such as methyl propionate, methyl pivalate, butyl pivalate, octyl pivalate and dimethyl oxalate; Amides such as dimethylformamide; And compounds containing S═O groups such as 1,3-propanesultone, glycol sulfite and divinyl sulfone.

The nonaqueous solvent may be used alone or in combination of two or more kinds. There is no particular limitation regarding the combination of nonaqueous solvents. Combinations include, for example, combinations of cyclic carbonates and linear carbonates, combinations of cyclic carbonates and lactones, and combinations of a plurality of cyclic carbonates and linear carbonates.

The electrolyte used in the present invention may be, for example, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiC (SO ₂ CF ₃ ) ₃ , LiPF ₄ (CF ₃ ) ₂ , LiPF ₃ (C ₂ F ₅ ) ₃ , LiPF ₃ (CF ₃ ) ₃ , LiPF ₃ (Iso-C ₃ F ₇ ) ₃ , and LiPF ₅ (Iso-C ₃ F ₇ ) do. These electrolytes may be used alone or in combination of two or more kinds. The electrolytes may be introduced into the non-aqueous solvent to provide an electrolyte at a concentration of electrolyte typically of 0.1 to 3 M, preferably 0.5 to 1.5 M.

The electrolytic solution of the present invention may, for example, mix the above-mentioned non-aqueous solvent; Dissolving the electrolyte in the mixture; It can be prepared by further dissolving at least one of the aforementioned cyclohexyl-halogenobenzenes in the mixture obtained.

The positive electrode active material is a composite metal oxide containing, for example, lithium and cobalt or nickel. The positive electrode active materials can be used alone or in combination. Composite metal oxides include, for example, LiCoO ₂ , LiNiO ₂ , and LiCo _1-X Ni _X O ₂ (0.01 <X <1). Such compounds can be used in any combination, for example, a combination of LiCoO ₂ and LiMn ₂ O ₄ , a combination of LiCoO ₂ and LiNiO ₂ , and a combination of LiMn ₂ O ₄ and LiNiO ₂ .

The positive electrode may be a positive electrode active material mentioned above, a conductive material such as acetylene black or carbon black, and a binder such as poly (tetra-fluoroethylene) (PTFE) or poly ( Vinylidene fluoride) (PVDF) is kneaded to obtain a positive electrode composition; The positive electrode mixture is applied onto a collector, for example a lath plate of aluminum foil or stainless steel; After drying and pressing the applied mixture, the compressed mixture may be prepared by heating in a vacuum at a temperature of about 50 to 250 ° C. for about 2 hours.

As the negative electrode active material, carbon materials (eg, artificial graphite and natural graphite) capable of absorbing and releasing lithium are used. It is preferable to use artificial graphite and natural graphite having a graphite crystal structure in which the lattice spacing of the lattice plane 002, that is, d ₀₀₂ is in the range of 0.335 to 0.340 nm. The negative electrode active materials can be used alone or in combination. The powder material, such as the carbon material, is preferably in combination with a binder, for example ethylene propylene diene terpolymer (EPDM), poly (tetrafluoroethylene) (PTFE), or poly (vinylidene fluoride) (PVDF). Used. There is no special limitation regarding the manufacturing method of the negative electrode. The negative electrode can be produced by a method similar to the method for producing the positive electrode.

There is no particular limitation on the structure of the nonaqueous lithium secondary battery of the present invention. The nonaqueous secondary battery may be, for example, a coin cell including a positive electrode, a negative electrode, and a single or a plurality of separators, or a cylindrical battery and a rectangular battery including a positive electrode, a negative electrode, and a separator roll. The separator may be a known material, for example, a microporous polyolefin membrane, a woven fabric, or a nonwoven fabric.

The lithium secondary battery of the present invention exhibits excellent cycle characteristics even when used under high end voltage charging conditions of greater than 4.2 V, in particular about 4.3 V. The discharge termination voltage may be at least 2.5 V, moreover at least 2.8 V. There is no particular limitation regarding the current value, and a constant current of 0.1 to 3 C is generally used at the time of discharge. The lithium secondary battery of the present invention may be charged and discharged within a wide temperature range of −40 to 100 ° C., preferably at 0 to 80 ° C.

The lithium secondary battery of the present invention may have a safety valve on the sealing plate to prevent an increase in the internal pressure. In addition, a notch may be provided in the cell container or gasket. One or more known safety devices can also be used, such as fuses, bimetal devices, and PTC devices, each of which is used as an overcurrent protection device.

If desired, the lithium secondary battery of the present invention may be wrapped in a battery pack by placing a plurality of batteries in series and / or in parallel. The battery pack may be a safety device, such as a PTC device, a thermostatic fuse, a fuse and / or a current blocking device, and additionally a safety circuit (i.e. monitor the voltage, temperature and current of the battery in the combined battery, Then a circuit capable of breaking the current).

Example 1

[Production of Non-Aqueous Electrolyte]

A non-aqueous electrolyte was prepared by dissolving 1 M LiPF ₆ in a nonaqueous solvent of EC: DEC = 3: 7 (volume ratio). 2.0% by weight of 1-fluoro-4-cyclohexylbenzene was further added to this non-aqueous electrolyte.

[Production of Lithium Secondary Battery and Measurement of Battery Characteristics]

LiCoO ₂ (positive electrode active material, 80% by weight), acetylene black (conductive material, 10% by weight), and poly (vinylidene fluoride) (binder, 10% by weight) were mixed. 1-methyl-2-pyrrolidone was further added to the obtained mixture. The resulting mixture was applied with aluminum foil, dried, pressurized and heated to produce a positive electrode.

Artificial graphite (negative electrode active material, 90% by weight), poly (vinylidene fluoride) (binder, 10% by weight) was mixed. 1-methyl-2-pyrrolidone was further added to the obtained mixture. The resulting mixture was applied with copper foil, dried, pressurized and heated to prepare a negative electrode.

A coin cell (diameter: 20 mm, thickness: 3.2 mm) was produced using the positive electrode, the microporous polypropylene membrane separator, and the nonaqueous electrolyte.

The coin cell was charged for 5 hours at a constant current of 0.8 mA to reach 4.2 V final voltage at room temperature (20 ° C.). The cell was then discharged at a constant current of 0.8 mA so that the termination voltage reached 2.7 V. The charging and discharging cycle was repeated.

The initial charge and discharge capacity was about the same as the capacity measured in the cell using 1M of LiPF ₆ and a solvent mixture (no additive) of EC / DEC (volume ratio 3/7) (see Comparative Example 1).

After 50 charge / discharge cycles, the discharge capacity retention rate with respect to the initial discharge capacity (100%) was 92.9%. Low temperature properties were also good.

Table 1 shows the manufacturing conditions and battery characteristics of the coin cell.

Example 2

The procedure of Example 1 was repeated except that a non-aqueous electrolyte was prepared and a coin cell was prepared except that 1-fluoro-4-cyclohexylbenzene was used as an additive at 5.0 wt%.

After 50 charge / discharge cycles, the discharge capacity retention rate was 91.4%.

Table 1 shows the manufacturing conditions and battery characteristics of the coin cell.

Example 3

The procedure of Example 1 was repeated except that 1-fluoro-4-cyclohexylbenzene was used as an additive at 0.5% by weight in the nonaqueous electrolyte to prepare a nonaqueous electrolyte and to manufacture a coin cell.

After 50 charge / discharge cycles, the discharge capacity retention rate was 90.5%.

Table 1 shows the manufacturing conditions and battery characteristics of the coin cell.

Comparative Example 1

A non-aqueous electrolyte was prepared by dissolving 1 M LiPF ₆ in a nonaqueous solvent of EC: DEC = 3: 7 (volume ratio). No cyclohexylbenzene compound was added to this nonaqueous electrolyte.

Using this non-aqueous electrolyte, a coin cell was produced in the same manner as in Example 1.

After 50 charge / discharge cycles, the discharge capacity retention rate was 82.6% of the initial discharge capacity.

Table 1 shows the manufacturing conditions and battery characteristics of the coin cell.

Example 4

The procedure of Example 1 was repeated except that the non-aqueous electrolyte was prepared and the coin cell was prepared except that 1-fluoro-2-cyclohexylbenzene was used as the additive at 2.0% by weight.

After 50 charge / discharge cycles, the discharge capacity retention rate was 92.4%. Low temperature properties were also good.                 

Table 1 shows the manufacturing conditions and battery characteristics of the coin cell.

Example 5

The procedure of Example 1 was repeated except that 1-fluoro-3-cyclohexylbenzene was used as an additive at 2.0 wt% in the nonaqueous electrolyte to prepare a nonaqueous electrolyte and produce a coin cell.

After 50 charge / discharge cycles, the discharge capacity retention rate was 92.0%.

Table 1 shows the manufacturing conditions and battery characteristics of the coin cell.

Example 6

The procedure of Example 1 was repeated except that 1-chloro-4-cyclohexylbenzene was used as an additive in a nonaqueous electrolyte at 2.0% by weight to prepare a nonaqueous electrolyte and produce a coin cell.

After 50 charge / discharge cycles, the discharge capacity retention rate was 89.1%.

Table 1 shows the manufacturing conditions and battery characteristics of the coin cell.

Example 7

The procedure of Example 1 was repeated except that 1-bromo-4-cyclohexylbenzene was used as an additive in a nonaqueous electrolyte at 2.0% by weight to prepare a nonaqueous electrolyte and produce a coin cell.

After 50 charge / discharge cycles, the discharge capacity retention rate was 88.9%.

Table 1 shows the manufacturing conditions and battery characteristics of the coin cell.

Comparative Example 2

The procedure of Example 1 was repeated except that a nonaqueous electrolyte was prepared and a coin-type battery was prepared except that fluorobenzene was used as an additive in a nonaqueous electrolyte at 5.0 wt%.

After 50 charge / discharge cycles, the discharge capacity retention rate was 82.9%.

Table 1 shows the manufacturing conditions and battery characteristics of the coin cell.

Comparative Example 3

The procedure of Example 1 was repeated except that the cyclohexylbenzene was used as an additive in a non-aqueous electrolyte at 5.0 wt% to prepare a non-aqueous electrolyte and produce a coin cell.

After 50 charge / discharge cycles, the discharge capacity retention rate was 83.1%.

Table 1 shows the manufacturing conditions and battery characteristics of the coin cell.

Example 8

Except for using natural graphite instead of artificial graphite as a negative electrode active material, the procedure of Example 1 was repeated to prepare a non-aqueous electrolyte and produce a coin-type battery.

After 50 charge / discharge cycles, the discharge capacity retention rate was 92.6%.

Table 1 shows the manufacturing conditions and battery characteristics of the coin cell.

Example 9

A LiCoO ₂ instead of the non-aqueous electrolytic solution was used _0.8 Co _0.2 O ₂ as the positive electrode LiNi active material was prepared and repeating the procedure of Example 1 to produce a coin type battery.

After 50 charge / discharge cycles, the discharge capacity retention rate was 91.0%.

Table 1 shows the manufacturing conditions and battery characteristics of the coin cell.                 

Example 10

The procedure of Example 1 was repeated except that LiMn ₂ O ₄ was used instead of LiCoO ₂ as the positive electrode active material to prepare a nonaqueous electrolyte and to manufacture a coin cell.

After 50 charge / discharge cycles, the discharge capacity retention rate was 92.4%.

Table 1 shows the manufacturing conditions and battery characteristics of the coin cell.                 

Positive electrode Negative additive
(Addition amount: weight%)
Electrolyte composition
(Capacity ratio) Initial discharge capacity
(Relative value) 50 cycle discharge capacity retention rate (%) Example 1 LiCoO ₂ Artificial graphite 1-fluoro-4-cyclohexylbenzene (2.0) 1M LiPF ₆
EC / DEC = 3/7 1.02 92.9 Example 2 LiCoO ₂ Artificial graphite 1-fluoro-4-cyclohexylbenzene (5.0) 1M LiPF ₆
EC / DEC = 3/7 1.01 91.4 Example 3 LiCoO ₂ Artificial graphite 1-fluoro-4-cyclohexylbenzene (0.5) 1M LiPF ₆
EC / DEC = 3/7 1.01 90.5 Comparative Example 1 LiCoO ₂ Artificial graphite none
1M LiPF ₆
EC / DEC = 3/7 1.00 82.6 Example 4 LiCoO ₂ Artificial graphite 1-fluoro-2-cyclohexylbenzene (2.0) 1M LiPF ₆
EC / DEC = 3/7 1.02 92.4 Example 5 LiCoO ₂ Artificial graphite 1-fluoro-3-cyclohexylbenzene (2.0) 1M LiPF ₆
EC / DEC = 3/7 1.02 92.0 Example 6 LiCoO ₂ Artificial graphite 1-chloro-4-cyclohexylbenzene (2.0) 1M LiPF ₆
EC / DEC = 3/7 1.01 89.1 Example 7 LiCoO ₂ Artificial graphite 1-bromo-4-cyclohexylbenzene (2.0) 1M LiPF ₆
EC / DEC = 3/7 1.01 88.9 Comparative Example 2 LiCoO ₂ Artificial graphite Fluorobenzene (5.0) 1M LiPF ₆
EC / DEC = 3/7 0.99 82.9 Comparative Example 3 LiCoO ₂ Artificial graphite Cyclohexylbenzene (5.0) 1M LiPF ₆
EC / DEC = 3/7 0.99 83.1 Example 8 LiCoO ₂ Natural graphite 1-fluoro-4-cyclohexylbenzene (2.0) 1M LiPF ₆
EC / DEC = 3/7 1.02 92.6 Example 9 LiNi _0.8 Co _0.2 O ₂ Artificial graphite 1-fluoro-4-cyclohexylbenzene (2.0) 1M LiPF ₆
EC / DEC = 3/7 1.14 91.0 Example 10 LiMn ₂ O ₄ Artificial graphite 1-fluoro-4-cyclohexylbenzene (2.0) 1M LiPF ₆
EC / DEC = 3/7 0.99 92.4

The present invention provides a lithium secondary battery having excellent battery characteristics in cycle characteristics, capacitance and storage characteristics of a battery.

Claims (7)

A lithium secondary battery comprising a positive electrode, a negative electrode made of artificial graphite or natural graphite, and a nonaqueous electrolyte having an electrolyte dissolved in a nonaqueous solvent, wherein the cyclohexylbenzene compound having the formula (I) is 0.1 to 20 wt. Lithium secondary battery containing%:

[Wherein X is a fluorine atom and the fluorine atom is attached at an arbitrary position]. delete The lithium secondary battery according to claim 1, wherein the cyclohexylbenzene compound having the formula (I) is 1-fluoro-4-cyclohexylbenzene. The lithium secondary battery according to claim 1, wherein the cyclohexylbenzene compound having the formula (I) is contained in 0.5 to 5% by weight in the nonaqueous electrolyte. The lithium secondary battery according to claim 1, wherein the nonaqueous solvent of the nonaqueous electrolyte comprises a combination of cyclic carbonates and linear carbonates, a combination of cyclic carbonates and lactones, or a combination of a plurality of cyclic carbonates and linear carbonates. The lithium secondary battery according to claim 1, wherein the nonaqueous solvent of the nonaqueous electrolyte contains vinylene carbonate. The lithium secondary battery according to claim 1, wherein the artificial graphite or natural graphite has a graphite crystal structure in which the lattice spacing d ₀₀₂ of the lattice plane 002 is in the range of 0.335 to 0.340 nm.